T cells with Fc receptors for different immunoglobulin isotypes (IgM, IgG, IgA and IgE) have been described. Likewise, T cell-derived soluble factors with similar specificity for isotypes have been demonstrated. We postulate, the existence of similar T cells and factors specific for IgD; preliminary evidence supports this assumption. Furthermore, we hypothesize, that these FcdeltaR+ T cells and soluble IgD-binding factors produced by them, have multiple and widespread regulatory functions on the B cell system as a whole. Capitalizing on the experience of one of us (K. Ishizaka), in the identification of Fc-epsilon-R+ T cells and of soluble IgE-binding factors, we plan to isolate analogous IgD-binding factors. We plan to study the regulatory function of FcdeltaR+ T cells and soluble factors on the regulation of the IgM/IgD ratio on the membranes of B lymphocytes and on antibody production "in vitro". We also plan to study the role of IgD-binding factors on proliferation and maturation of B cells stimulated "in vitro" with anti-IgM antibodies. We shall also study the effect of the same factors on the isotype switch of B cells induced "in vitro" by LPS. In the last two series of experiments, the effect of T cell-derived IgD-binding factors will be compared with that of different lymphokines, produced by T cells, that are known to affect the same processes of B cell proliferation and differentiation.